1. Field of the Invention
The present invention relates to a ferroelectric memory device, a semiconductor memory device, and the like to which a ferroelectric capacitive element is applied.
2. Description of the Related Art
Recently, various techniques of applying a capacitive element (capacitor) using a ferroelectric thin film to a semiconductor memory device and an electronic device, have been known to public.
When the ferroelectric thin film is used as a capacitive element, it is usually important to utilize the characteristics of the thin film with efficiency. More specifically, the ferroelectric thin film has to be formed such that it has the characteristics of increasing the residual polarization, reducing the dielectric constant, lowering the resistant voltage, lengthening the retaining time, resisting the repetitive inversion of polarization, and the like, and these characteristics have to be unchanged in the integration process.
The above characteristics vary with materials of electrodes, conditions of interface, crystal layer of ferroelectrics, size of grains, etc. In particular, if, as shown in FIG. 8A, an adhesive layer 2, a lower electrode layer 3, a ferroelectric thin film 4, and an upper electrode layer 5 are formed in this order on a substrate 1, platinum (Pt), gold (Au), palladium (Pd), or the like is used for the materials of the electrode layers in order to prevent them from being oxidized, since there is a step of thermally treating the ferroelectric thin film 4 in an oxygen atmosphere.
An attempt to substitute the above electrode materials for other metallic materials, especially conductive oxides, has been made in view of their characteristics and variations in the characteristics. It is known that, if ITO (InSuO) is used for the electrode material, the hysteresis characteristics of a ferroelectric serving as a capacitive element is improved and, in other words, a square ratio is improved. Since, however, the mechanism of the ferroelectric capacitive element has not yet been clarified specifically, it is not applied to electronic components and the like.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2-248089 discloses using a perovskite conductive oxide film as an electrode to prevent oxygen from diffusing, and Jpn. Pat. Appln. KOKAI Publication No. 4-85878 teaches using ITO as a material for an electrode to prevent oxygen from diffusing at the time of oxidization. Further, Jpn. Pat. Appln. KOKAI Publications Nos. 4-206869 and 4-367211 propose a technique of improving in crystallinity and characteristic by using a conductive oxide such as ITO and RuO as an electrode.
If, however, the above prior art capacitive element is constituted by CVD or the like, there occurs a problem in which the characteristics of the ferroelectrics of the capacitive element deteriorates in a reductive gas containing H.sub.2 serving as an atmospheric gas.
A generally-used memory device has a multilayer structure as shown in FIG. 8B. The multilayer structure is based on the CMOS or NMOS structure and includes a ferroelectric formed on the silicon substrate. Such a structure will now be described by way of example.
First, a MOSFET 6, which is representative of a pass gate type transistor, is formed on the silicon substrate to function as a gate of the memory device. Then, a memory capacitor 7 connected to the gate is constituted of a platinum electrode, a ferroelectric thin film, and a platinum electrode, a protection film (passivation layer) 8 is constituted of an insulator formed on SiO.sub.2 layers 8a, and a third electrode 10 is formed. SiO.sub.2 layer 8a is on the silicon substrate 8b. When the memory capacitor 7 is formed, the MOSFET 6 is exposed to charged particles included in plasma and various contaminations, resulting in problems of shift in threshold voltage Vth or deterioration of sub-threshold characteristics and increase in leak current.
In order to resolve the above problems, anneal treatment is carried out for a predetermined period of time at, for example, 400.degree. to 450.degree. C. in an N.sub.2 atmosphere containing 2 to 10% H.sub.2. However, the conventional ferroelectric capacitive element is constituted on the basis of an oxide film and precious metal such as platinum which does not form an oxide. In general, an oxide is reduced into a metal atom in the atmospheric gas (reductive gas) of the anneal treatment.
A number of defects in crystal (lack of oxygen molecules, etc.) occur in the vicinity of an interface between the platinum and ferroelectric, and hydrogen molecules (H.sub.2) are easy to diffuse in this interface. For this reason, the ferroelectric is reduced and its surface is metallized, with the result that the metallized surface and platinum contact each other at the interface. Since no alloy is formed by the platinum and metal, the contact therebetween is very poor. Therefore, when the third electrode 10 is formed, exfoliation occurs at the interfaces between a via hole of the passivation layer 8, the upper electrode 3, and the lower electrode 5 because of their great stress. This exfoliation not only destroys the memory cells of the memory device but also forms an air layer in the interfaces to adversely affect the electrical characteristics of the device.
None of the foregoing prior art publications refer to any solution to the above problems.